Cone and socket joints have been in use for many years for the assembly of glass or quartz apparatus. They can be used to connect together lengths of tubing or pieces of apparatus and are usually also made of glass or quartz but are sometimes made of other material such as metal or plastics. These cone and socket joints are standardised in many countries. For example in the Federal Republic of Germany the relevant specifications are DIN Nos. 12242, 12248 and 12249. Cone and socket joints are also the subject of international recommendations, for example those of the International Standards Organization No. P383-1964. These provide a union which is characterised by universal interchangeability, ease of assembly and dismantling, stability, cleanliness and resistance to chemicals. They are primarily known by chemists for their use in laboratory glassware such as is commonly employed in organic chemistry research laboratories, but the same system is now used for glass to metal or metal to metal or quartz to quartz joints and the term "laboratory glassware" as used herein is defined to include any joint of this kind.
The standard cone and socket joint comprises a male member consisting of a hollow truncated cone disposed at the end of a tube or tubular limb of a piece of apparatus or component thereof with a prominent annular shoulder formed between the cone and the adjacent tubing. The female member consists of a socket internally dimensioned to accommodate the male member and possessing a circumferentially thickened external strengthening lip around the mouth, the female member extending integrally into another length of tubing or tubular limb of a piece of apparatus or of a component thereof. The outer surface of the cone and the inner surface of the socket, which mate intimately on assembly of the joint, are commonly ground or in some forms polished, and the gradient of the matching tapers on the cone and socket is usually 1:10, less commonly 1:5.
In spite of the advantages of the familiar cone and socket joints, they are not fully satisfactory for all situations encountered in laboratory practice. Where they are to be used in apparatus working under reduced pressure, care must be taken to ensure that they are made effectively air tight. Cone and socket joints are conventionally made air tight by applying vacuum grease to the mating surfaces. Although this procedure provides a satisfactory seal it cannot be used in cases where the unavoidable contact between the materials being worked and the vacuum grease leads to adverse effects. Furthermore, when cone and socket joints are to be used in systems working at above atmospheric pressure, the same care as above must be taken to ensure sufficient sealing and in addition the joint must be prevented from opening spontaneously under the action of the internal pressure. Attempts have been made to meet these requirements by using adhesive substances instead of grease on the mating surfaces, but this does not afford a sufficiently reliable guarantee against inadvertent separation of the joint under pressure. Furthermore a joint treated in this way cannot readily be dismantled when required.
Another way of securing a cone and socket joint is by means of forked clamps which grip behind the annular shoulder of the cone and behind the strengthening lip of the socket and which are engaged by a loaded spring to urge the cone and socket together. However, there is not reliable guarantee that the joint will not open under abnormally high internal pressures. The same is true of the familiar spring wire clips which are also fastened under tension between the annular shoulder of the cone and the strengthening lip of the socket. Other similar devices employing spring action to prevent separation of the joint suffer from the same disadvantages.
In a non-standard form of cone and socket joint which is described in U.S. Pat. No. 3,140,097, the male member possesses an annular groove in which an O-ring of resilient material is seated, the groove being positioned adjacent to the ground conical surface at its greatest diameter. On insertion of the cone into the socket, the O-ring flattens and seals against the smooth (i.e. unground) inner wall of an extension of the socket. The disadvantages of this form of joint are that it is not in accordance with the universal standard form, and also that it can only be used to advantage in systems working at reduced pressure since positive internal pressure promotes separation of the two members.
A coupling has been proposed in U.K. Specification No. 1,174,948 in which a resilient rubber washer is sandwiched between a male threaded collar which locates behind the reinforcing flange of the socket portion of the joint and a female threaded cap which fits over the cone portion of the joint behind the rubber washer. However, with this arrangement the rubber washer will normally be exposed to a torsional deforming pressure or couple which leads to rapid wear and may result in a poor seal. Furthermore, considerable variations commonly occur in the external diameter of the socket immediately behind the strengthening lip, and a collar having a fixed diameter will often not locate properly behind the strengthening lip.
In German Gebrauschmuster No. 7633564 the inventor herein has described a coupling in which a first annular member is positioned about the cone portion of the joint immediately behind the shoulder and has a male threaded end. A second annular member is positioned about the socket portion of the joint, is engaged behind the external strengthening lip around the mouth of the socket portion, and has a female threaded end which may be screwed onto the male threaded end of the first annular member to hold the cone portion and the socket portion of the joint together. The first annular member is formed with a seat which engages with an elastomeric sealing ring which is positioned about the annular shoulder so as to exert an axial load thereon to compress the elastomeric sealing ring into the external line of junction corresponding to the outermost circumference of contact between the cone portion and the socket portion of the assembled joint. While the joint constructions described and claimed in the said Gebrauschmuster provide desirable improvements in joint coupling technology, the invention herein has found it desirable to make the joint self-centering and to provide a more positive location behind the cone and socket joint.